As intermediates in the synthesis of various useful compounds such as medicine and food flavorings, there have been known various optically active carboxylic acids. Representative examples of the methods for the preparation of these optically active carboxylic acids include a method which comprises the preparation from natural substances, a method which comprises the use of microorganism, and a method which comprises the use of an asymmetric hydrogenation catalyst. Among these methods, the method which comprises the use of an asymmetric hydrogenation catalyst attracts the most attention from the standpoint of availability of raw materials, reaction controllability and cost.
Beginning with the proposal for the process for the preparation of an asymmetrically-hydrogenated compound having a high optical purity by the selective use of a complex comprising as a ligand a rhodium atom and an optically active phosphine as an asymmetric hydrogenation catalyst, many studies of the use of a complex comprising a transition metal atom and optically active phosphine as an asymmetric hydrogenation catalyst have been reported.
For example, J. Org. Chem., Vol. 52, No. 14, pp. 3174-3176 (1987) discloses a technique for the preparation of an optically active carboxylic acid which comprises the asymmetric hydrogenation of an unsaturated carboxylic acid in the presence of a 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl-ruthenium complex catalyst (hereinafter referred to as "BINAP-Ru catalyst").
This method enables the preparation of a carboxylic acid having a high optical purity under milder conditions than the conventional methods and thus is useful. However, this method is disadvantageous in that the reaction must be effected at a hydrogen pressure of 1 to 100 atm. over tens of hours.
Further, Tetrahedron Lett., vol. 34, pp. 2351-2354 (1993) discloses a technique for the asymmetric hydrogenation of tiglic acid in the presence of a catalyst comprising 2,2'-bis(dicyclohexylphosphino)-6,6'-dimethyl-1,1'-biphenyl (hereinafter referred to as "BICHEP") and p-cymene coordinated to a ruthenium atom.
Moreover, JP-A-63-310847 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a technique for the preparation of an optically active alcohol which comprises the asymmetric hydrogenation of ketonic acids in the presence of BINAP-Ru catalyst.
This method enables the preparation of a carboxylic acid having a high optical purity under milder conditions than the conventional methods and thus is useful. However, this method is disadvantageous in that the reaction must be effected at a hydrogen pressure of 5 to 40 atm. over tens of hours.
On the other hand, a complex comprising ruthenium as a transition metal atom and an unsaturated acid as a ligand is disclosed in J. Am. Chem. Soc., Vol. 113, No. 2, pp. 589-594 (1991). However, this reference discloses no idea of the use of such a complex as a catalyst for an asymmetric hydrogenation reaction. Further, this reference merely discloses that such a complex comprises two unsaturated acid molecules or one unsaturated acid molecule and one saturated acid molecule coordinated to ruthenium atom.